Recently, for enabling transfer of micropatterns having a size of 40 nm or less, EUV exposure technology has become considered to be a promising one in place of existing ArF exposure technology that uses ArF excimer laser light having a wavelength of 193 nm. In EUV exposure technology, used is EUV (Extreme Ultra-Violet) light having a shorter wavelength than ArF excimer laser light, as the exposure light therein. Here, the EUV light includes soft X-ray and vacuum UV light, and is concretely light having a wavelength of from 0.2 to 100 nm or so. At present, as the exposure light, EUV light having a wavelength of 13.5 nm or so is mainly investigated.
In EUV lithography (EUVL) technology, a reflective photomask is used. The reflective photomask comprises a multilayer reflective film and an absorber layer as formed in this order on a substrate, in which a part of the absorber layer is removed. The absorber layer is formed in a prescribed pattern. EUV light incident on the reflective photomask is absorbed in area where the absorber layer exist but is reflected by the multilayer reflective film in the other area where the absorber layer does not exist whereby an image is formed on the surface of the exposed material by an optical system. In that manner, the pattern of the absorber layer is transferred to the surface of the exposed material.
The multilayer reflective film has a periodical structure of some types of layers each having a different refractive index and repeatedly stacked on a substrate in a prescribed order. For example, the multilayer reflective film comprises, as stacked alternately and repeatedly therein, Mo layers as low-refractivity layers and Si layer as high-refractivity layers.
In cases where the multilayer reflective film is contaminated with foreign substances during stacking thereof, or in cases where defects (e.g., foreign substances, flaws, pits) exist in the surface of the substrate on which the multilayer reflective film is formed, the periodical structure of the multilayer reflective film would be disordered to give defects (so-called phase defects) in the multilayer reflective film. Such defects, if formed, would bring about a problem in that the pattern of the reflective photomask could not be faithfully transferred to a wafer. Technically, it is extremely difficult to absolutely remove the defects from the multilayer reflective film (e.g., see Non-Patent Document 1).
Given the situation, investigated is a technique of controlling the position and the direction of the pattern of the absorber layer in accordance with the position of the defects in the multilayer reflective film (e.g., see Non-Patent Document 2).
In addition, for accurately identifying the position of the defects in the multilayer reflective film, there is proposed a technique of previously forming a fiducial mark on the surface of the substrate on which the multilayer reflective film is formed (e.g., see Patent Document 1). The fiducial mark is transferred to the multilayer reflective film, and based on the position of the transferred fiducial mark as the reference position, the position of the defects in the multilayer reflective film can be identified.
Apart from the above, there is also proposed a technique of correcting the defects in the multilayer reflective film by identifying the position of the defects in the multilayer reflective film (e.g., see Patent Document 2). Patent Document 2 says that, when an absorber layer is formed on the multilayer reflective film therein, a fiducial mark is formed on the absorber layer but is not formed on the substrate nor the multilayer reflective film.